This invention relates to metal detectors, and more particularly, to a discriminating metal detector that compensates for ground minerals while at the same time rejecting trash and detecting valuable objects in the high mineral ground.
The first metal detectors used by amateur and professional treasure hunters were of the type that detected all metals alike. This meant that treasures such as coins, rings, jewelry, and other valuables would give an increase in the audio signal as would trash such as nails, tin foil, pull-tabs and other junk. Next came the discriminating metal detectors that could separate the treasure from the trash. The treasure or desirable metals would give an increase in audio tone, while the trash would give a decrease or no change in sound level. However, the discriminating detector while a great improvement over the all metal detector in high trash search areas still encountered difficulties when the ground contained a high degree of mineralization in the search area. This resulted in the latter detector circuitry to emit changes in the audio tone due to soil minerals, or in other words false signals.
An unique discriminating metal detector of the latter type is disclosed in U.S. Pat. No. 3,896,371 which can separate the treasure from the trash where the ground has a low mineral content. However, the natural resonance of the LC circuit in its search head is shifted when the ground minerals are high in the search area. This shift in frequency causes the detector to emit false signals. Ground minerals are defined as soil containing a powdered, compressed, and sintered magnetic material having high resistivity, consisting chiefly of ferric oxide combined with one or more metals.